Rocks: Igneous, Sedimentary & Metamorphic – Key Vocabulary

Comprehensive set of vocabulary flashcards covering key terms related to igneous, sedimentary, and metamorphic rocks, their formation processes, textures, compositions, and associated geological concepts.

Igneous Rock

Rock formed by the solidification of molten material (magma or lava).

Sedimentary Rock

Rock created from the accumulation, compaction, and cementation of sediments or precipitation of minerals from solution.

Metamorphic Rock

Rock that has been altered in mineralogy, texture, or chemistry by heat, pressure, or fluids without melting.

Magma

Molten or semi-molten rock beneath Earth’s surface containing crystals, dissolved gases, and volatiles.

Lava

Magma that reaches Earth’s surface and flows or erupts from a volcano.

Decompression Melting

Partial melting of mantle rock caused by a drop in pressure without an increase in temperature.

Flux-Induced Melting

Melting triggered when volatiles (e.g., water) lower the melting point of hot mantle rock.

Intrusive (Plutonic) Rock

Igneous rock that crystallizes slowly beneath the surface, forming large crystals.

Extrusive (Volcanic) Rock

Igneous rock that cools rapidly at the surface, producing fine-grained or glassy textures.

Aphanitic Texture

Very fine-grained igneous texture; crystals are too small to see unaided.

Phaneritic Texture

Coarse-grained igneous texture with interlocking crystals large enough to identify by eye.

Porphyritic Texture

Igneous texture with large phenocrysts embedded in a finer groundmass.

Glassy Texture

Texture produced when lava cools so quickly that ions are frozen in disorder, forming natural glass.

Vesicular Texture

Igneous texture containing many gas bubble cavities (vesicles).

Pyroclastic Texture

Fragmental texture made of volcanic ash, lapilli, and blocks ejected during explosive eruptions.

Phenocryst

A large, conspicuous crystal in a porphyritic igneous rock.

Groundmass

The finer-grained matrix surrounding phenocrysts in a porphyritic rock.

Vesicle

Spherical or elongated void left by expanding gas bubbles in volcanic rock.

Pumice

Light, frothy, vesicular volcanic glass formed from highly gas-charged felsic lava.

Obsidian

Dark, natural volcanic glass with a conchoidal fracture and glassy texture.

Scoria

Dark, highly vesicular mafic volcanic rock heavier than pumice.

Granite

Coarse-grained, felsic intrusive rock dominated by quartz and potassium feldspar.

Rhyolite

Fine-grained, felsic extrusive equivalent of granite.

Diorite

Coarse-grained, intermediate intrusive rock composed mainly of plagioclase and amphibole.

Andesite

Fine-grained, intermediate extrusive rock commonly found in volcanic arcs.

Gabbro

Coarse-grained, mafic intrusive rock composed of pyroxene, olivine, and plagioclase.

Basalt

Fine-grained, mafic extrusive rock; Earth’s most common volcanic rock.

Peridotite

Ultramafic intrusive rock rich in olivine and pyroxene; dominant in the upper mantle.

Komatiite

Rare ultramafic extrusive rock formed from very hot, low-viscosity lava in Earth’s early history.

Felsic

Igneous composition rich in silica, quartz, and feldspar; light-colored and low-density.

Intermediate (Andesitic) Composition

Igneous composition between felsic and mafic with moderate silica content.

Mafic

Igneous composition rich in magnesium and iron; dark-colored and denser.

Ultramafic

Very low silica igneous composition dominated by olivine and pyroxene; darkest and densest.

Viscosity (of Magma)

Resistance to flow; increases with higher silica and lower temperature.

Bowen’s Reaction Series

Sequence describing the order in which minerals crystallize from cooling magma.

Dark Silicates

Iron- and magnesium-rich minerals such as olivine, pyroxene, amphibole, and biotite.

Light Silicates

Silica-rich minerals including quartz, muscovite, and feldspars.

Olivine

High-temperature, green, magnesium-iron silicate common in ultramafic rocks.

Pyroxene

Group of dark silicate minerals common in mafic and ultramafic rocks.

Amphibole

Dark, elongated silicate mineral group typical of intermediate rocks.

Biotite Mica

Dark mica mineral rich in iron and magnesium; cleavage in thin sheets.

Quartz

Hard, silica mineral (SiO₂) common in felsic rocks; resists weathering.

Feldspar

Most abundant mineral group in Earth’s crust; includes plagioclase and alkali feldspars.

Plagioclase Feldspar

Sodium- to calcium-rich feldspar series common in all igneous compositions.

Alkali Feldspar

Potassium- and sodium-rich feldspars (orthoclase, microcline, sanidine).

Magmatic Differentiation

Process by which magma evolves and produces varying rock types through crystallization and removal of minerals.

Pluton

General term for a body of intrusive igneous rock formed at depth.

Batholith

Massive, discordant pluton covering >100 km² at the surface; e.g., Sierra Nevada.

Sill

Sheet-like intrusive body that intrudes parallel to existing rock layers.

Dike

Tabular intrusive body that cuts across pre-existing structures.

Laccolith

Mushroom-shaped intrusion that domes overlying strata.

Volcanic Neck

Erosional remnant of solidified magma within a volcanic conduit.

Conglomerate

Clastic sedimentary rock with rounded gravel-size clasts (>2 mm).

Breccia

Clastic sedimentary rock with angular gravel-size clasts (>2 mm).

Sandstone

Clastic sedimentary rock composed chiefly of sand-size grains (0.0625–2 mm).

Shale

Very fine-grained clastic rock composed mainly of clay minerals; exhibits fissility.

Siltstone

Fine-grained clastic rock dominated by silt-size particles; lacks shale fissility.

Clastic Sedimentary Rock

Rock formed from mechanical weathering debris lithified into solid rock.

Chemical Sedimentary Rock

Rock formed by precipitation of minerals from water solutions (e.g., limestone, rock salt).

Organic Sedimentary Rock

Rock derived from accumulated plant or animal debris (e.g., coal, some limestones).

Carbonate Rock

Sedimentary rock composed chiefly of carbonate minerals such as calcite or dolomite.

Evaporite

Chemical sedimentary rock formed by evaporation of saline water, yielding halite, gypsum, etc.

Stratification

Layering or bedding in sedimentary rocks formed by successive deposition.

Lamination

Thin (<1 cm) bedding in very fine-grained sedimentary rocks.

Cross Bedding

Inclined sedimentary layers deposited by currents or wind within horizontal strata.

Graded Bedding

Sedimentary bed showing progressive grain-size decrease from bottom to top.

Ripple Marks

Small wave-like sedimentary structures produced by water or wind movement.

Metamorphism

Solid-state alteration of rock due to heat, pressure, and/or fluids.

Protolith

The original, unmetamorphosed parent rock before metamorphism.

Contact Metamorphism

Thermal metamorphism of country rock adjacent to an igneous intrusion; non-foliated textures common.

Regional Metamorphism

Large-scale metamorphism associated with mountain-building; produces foliated rocks.

Foliation

Planar arrangement of mineral grains or structural features in metamorphic rocks.

Non-Foliated Rock

Metamorphic rock lacking planar fabric; minerals grow in randomly oriented crystals.

Slate

Low-grade, fine-grained foliated metamorphic rock derived from shale; exhibits slaty cleavage.

Phyllite

Low- to medium-grade foliated rock with satiny sheen and wavy foliation.

Schist

Medium-grade foliated rock with visible platy minerals (schistosity).

Gneiss

High-grade foliated rock with compositional banding of light and dark minerals.

Marble

Non-foliated metamorphic rock formed from limestone or dolostone; composed of calcite. 

Quartzite

Hard, non-foliated metamorphic rock formed from quartz sandstone.

Hornfels

Fine-grained, non-foliated contact metamorphic rock produced by baking of various protoliths.

Anthracite

Hard, high-grade metamorphic coal with high carbon content and luster.

Peat

Partially altered plant material; first stage in coal formation.

Lignite

Soft, brown coal formed from compacted peat.

Bituminous Coal

Soft, black coal of intermediate rank in the coal series.

Confining Pressure

Uniform pressure applied in all directions due to overlying rock burden.

Differential Stress

Unequal pressure in different directions that deforms rocks, producing foliation.

Chemically Active Fluids

Hot, ion-rich fluids (often water and volatiles) that enhance metamorphic reactions.